A numerical study on the shedding frequency of sheet cavitation

The last decades there is a strong interest in predicting cavitation dynamics as it is a prerequisite in order to predict cavitation erosion. Industrial applications require accurate results in an acceptable time span and as a result there is a focus on large scale dynamics. In this paper the RANS equations are used to investigate the shedding frequency of sheet cavities in two-dimensional simulations. First a verification study is made for the NACA 0015 in 6 degrees angle of incidence. A grid sensitivity study is conducted in wetted flow and in steady (non-shedding) cavitating condition (σ=1.6). Then an investigation is conducted in order to capture the shedding frequency. The results show that only when a correction for turbulent viscosity at the cavity-water interface is used it was possible to capture the shedding frequency as found in other numerical studies. Furthermore, a validation study is conducted on a NACA66-312 α=0.8 for two different angles of attack. The obtained results are compared and validated with the experimental data from Leroux et al. They indicate that the 2D shedding frequency predicted by the numerical simulations is in good agreement with the frequency obtained in the experiment

Numerical study of cavitation on a NACA0015 hydrofoil: solution verification

Abstract. The present paper analyses a series of Computational Fluid Dynamic simulations of the cavitating flow around a two-dimensional NACA0015 foil. The foil is placed at 6◦ angle of attack and the cavitation number is 1.1. Two mesh designs, namely a block-structured topology and an unstructured topology, are compared; additionally, grid refinements and time step refinements are carried out. Solution Verification is addressed with calculation of the discretization error and the numerical uncertainty. The numerical uncertainty for the average lift coefficient is found to be large, up to 15%. The reason is the difficulty of achieving a grid independent solution: with very fine meshes, the flow shifts from an attached, oscillating sheet cavity pattern to a regime dominated by shedding of cavity clouds. On the other hand, neither the time resolution nor the choice of grid topology influence largely the flow pattern; instead, they only lead to differences in the maximum and minimum cavity size

Assessment of Cavitation Erosion With a URANS Method

An assessment of the cavitation erosion risk by using a contemporary unsteady Reynoldsaveraged Navier-Stokes (URANS) method in conjunction with a newly developed postprocessing procedure is made for an NACA0015 hydrofoil and an NACA0018-45 hydrofoil, without the necessity to compute the details of the actual collapses. This procedure is developed from detailed investigations on the flow over a hydrofoil. It is observed that the largescale structures and typical unsteady dynamics predicted by the URANS method with the modified shear stress transport (SST) k-x turbulence model are in fair agreement with the experimental observations. An erosion intensity function for the assessment of the risk of cavitation erosion on the surface of hydrofoils by using unsteady RANS simulations as input is proposed, based on the mean value of the time derivative of the local pressure that exceeds a certain threshold. A good correlation is found between the locations with a computed high erosion risk and the damage area observed from paint tests

Pans simulations: low versus high reynolds number approach

Different approaches for specifying the ratio of modelled-to-total dissipation ( fε) in the PANS model, based on the k −ω SST model, are evaluated for different ratios of the modelled-to-total kinetic energy, fk. Based on theoretical reasoning it is argued that applying fε = a · fk should have little effect, and that fε = fk is not expected to improve the results. This is confirmed by applying the approaches to a turbulent channel flow at Reτ = 395 and 180, and comparing the results to the often used ‘high Reynolds number’ approach ( fε = 1.0). Reducing fε leads to a reduction in range of scales in the flow; dissipation is allowed at larger scales and therefore smaller scales are suppressed

Numerical Study of Cavitation on a NACA0015 Hydrofoil: Solution Verification

The present paper analyses a series of Computational Fluid Dynamic simulations of the cavitating flow around a two-dimensional NACA0015 foil. The foil is placed at 6 • angle of attack and the cavitation number is 1.1. Two mesh designs, namely a block-structured topology and an unstructured topology, are compared; additionally, grid refinements and time step refinements are carried out. Solution Verification is addressed with calculation of the discretization error and the numerical uncertainty. The numerical uncertainty for the average lift coefficient is found to be large, up to 15%. The reason is the difficulty of achieving a grid independent solution: with very fine meshes, the flow shifts from an attached, oscillating sheet cavity pattern to a regime dominated by shedding of cavity clouds. On the other hand, neither the time resolution nor the choice of grid topology influence largely the flow pattern; instead, they only lead to differences in the maximum and minimum cavity size